An appositive is a pronoun, noun, or noun phrase that identifies, explains, or describes an adjacent pronoun, noun, or noun phrase. That is, an appositive clarifies the meaning or significance of the pronoun, noun, or noun phrase that is accompanies.

Sometimes for more emphasis upon the qualities of a person an appositive will begin a sentence.

e.g. A living symbol, Pearl is the spiritual and physical connection for Hester Prynne and Arthur Dimmesdale.

In order to rewrite the above-mentioned sentences with an appositive, the writer can change the second sentence into an appositive phrase:

Insomniacs, people who have trouble falling asleep, have probably seen the movie twenty times.

Notice that before the revision, the noun insomniac has been repeated. So, it can be eliminated and an appositive phrase can be used for the clarifying explanation instead.